The ultimate goal of this project is to understand the oligomeric structure and structure-function relationships of the human hepatic asialoglycoprotein receptor (ASGP-R). This receptor serves as a convenient model to define general principles of receptor biosynthesis, subunit assembly, targeting, ligand binding, receptor-mediated endocytosis, and receptor cycling at the molecular level. In addition, because of its tissue and ligand specificity, the ASGP-R has the potential to be being the target and entry point for specific therapeutic agents directed to the liver. since genes can be specifically targeted to liver cells by coupling them to the ASGP-R ligand, this receptor also affords the potential for organ-specific gene therapy. The aims of this proposal are threefold. First, we will determine the oligomeric structure and stoichiometry of the H1 and H2 subunits of the ASGP-R, using chemical cross-linkers and affinity isolation techniques. Second, we aim to define and analyze the domains of H1 and H2 that complement function defects in the other subunit. We will do this by constructing deletion mutants and chimeras of H1 and H2, expressing them alone or together with intact subunits in NIH 3T3 fibroblasts, and testing for receptor function. Finally, we will characterize the effects of ligand occupancy on receptor internalization and trafficking, seeking in particular to elucidate the biochemical and biophysical mechanisms by which ligand occupancy "triggers" an acceleration in the internalization rate of receptor-ligand complexes.